Si RECESS METHOD IN HKMG REPLACEMENT GATE TECHNOLOGY

ABSTRACT

The present disclosure relates to a method of embedding an ESF3 memory in a HKMG integrated circuit that utilizes a replacement gate technology. The ESF3 memory is formed over a recessed substrate which prevents damage of the memory control gates during the CMP process performed on the ILD layer. An asymmetric isolation zone is also formed in the transition region between the memory cell and the periphery circuit boundary.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced exponential growth over the last few decades. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. One advancement implemented as technology nodes shrink, in some IC designs, has been the replacement of the typically polysilicon gate electrode with a metal gate electrode to improve device performance with the decreased feature sizes.

Super-flash technology has enabled designers to create cost effective and high performance programmable SOC (system on chip) solutions through the use of split-gate flash memory cells. The aggressive scaling of the third generation embedded super-flash memory (ESF3) enables designing flash memories with very high memory array density.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIG. 1 illustrates a cross sectional view of an integrated circuit with a pair of flash memory cells disposed on a recessed substrate and a high-k metal gate (HKMG) circuitry disposed on an un-recessed substrate according to some embodiments of the present disclosure.

FIG. 2 illustrates a flow diagram of some embodiments of a method for forming an integrated circuit according to the present disclosure.

FIG. 3 illustrates a flow diagram of some embodiments of a detailed method for forming an integrated circuit till a certain stage of its formation, according to the present disclosure.

FIGS. 4A-4K illustrate embodiments of cross sectional images of a step by step method of formation of a recessed substrate surface with isolation zones, according to the present disclosure.

FIGS. 5A-5D illustrate cross sectional images of an integrated circuit during different stages of a replacement gate process, according to the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

High-k metal gate (HKMG) technology has become one of the front-runners for the next generation of CMOS devices. This technology incorporates a high-k dielectric, which reduces leakage and improves the dielectric constant. To help with fermi-level pinning and to allow the gate to be adjusted to low threshold voltages, a metal gate is used instead of a polysilicon gate. By combining the metal gate and low-k dielectric, HKMG technology reduces gate leakage, thereby increasing the transistor capacitance and allowing chips to function with reduced power needs. HKMG process technology involves two distinctly different integration flows, namely traditional gate and replacement gate. In a traditional gate approach, the transistor's gate stack is fabricated first, followed by the source and drain, very much like traditional CMOS transistor architecture. A replacement gate process is just the opposite, where a sacrificial gate stack is stripped away to form a recess after source and drain regions have been formed, and a replacement gate is fabricated in the recess after formation of the source and drain regions.

Flash memory is an electronic non-volatile computer storage medium that can be electrically erased and reprogrammed. Super-flash is a highly reliable and versatile type of NOR flash memory and NOR is a form of random access nonvolatile memory that allows electronic systems to retain information when the power source is turned off. Through scaling and evolution, the memory cell retains its split-gate structure, poly-to-poly Fowler-Nordheim (FN) tunneling erase, and Source-Side Hot Channel Electron (SS HCE) injection programming. To further enhance the scaling of the split-gate memory cell, a coupling gate/control gate (CG) is introduced in Super Flash's third generation (ESF3) technology.

In conventional process flows, silicon nitride (SiON) ESF3 memory can only be embedded in traditional gate HKMG technology and is not compatible with replacement HKMG technology. The reason for this incompatibility stems from a chemical mechanical polishing (CMP) process used in the replacement gate process. This CMP process is performed right before the sacrificial gate stack is removed, and planarizes the structure until the top surface of the sacrificial gate stack is reached. If SiON ESF3 memory is embedded in the replacement gate process, the above mentioned CMP process will damage the CGs of the ESF3 memory cells because the CGs of the ESF3 memory stand taller than the sacrificial gate stack.

Accordingly, the present disclosure relates to a new processing method which allows embedding SiON ESF3 memory in the replacement gate HKMG technology. Here, a portion of the Si substrate where the ESF3 memory would be embedded is recessed to a depth below the height of the original substrate, such that the HKMG circuitry is formed on an un-recessed Si substrate and the ESF3 memory is formed on a recessed Si substrate. This recession will align the top surfaces of the CGs of the ESF3 memory and the sacrificial gate stack of the HKMG transistor, horizontally. In other words the top surfaces of the CGs and the sacrificial gate will become co-planar. Thus, the Si recess would embed the ESF3 memory at a lower height level with respect to the HKMG circuitry so that the CMP process would stop at the top surface of the CGs and not damage them.

FIG. 1 illustrates a cross sectional view of an integrated circuit 100 with a pair of flash memory cells disposed on a recessed substrate and a high-k metal gate (HKMG) circuitry disposed on an un-recessed substrate according to some embodiments of the present disclosure. Integrated circuit 100 resides on a Si substrate 102 which has a recessed region 101 and an un-recessed region 103, with a transition region 105 there between. Top surface of the un-recessed region 103 or the original top surface of the Si substrate 102 is represented by horizontal line 102 a. Top surface of the recessed region 101 is represented by horizontal line 102 b.

A pair of adjacent flash memory cells 132, 134 resides on the recessed substrate region 101. The flash memory cells 132, 134 comprise control gate structures 109 a, 109 b, wherein each control gate structure comprises a thicker CG (e.g., 110 a) which is disposed above a thinner yet wider floating gate (FG) (e.g., 108 a). The memory cells share a common erase gate (EG) 112 and common source/drain region 106. Select gates (SG) 116 a, 116 b are disposed on the opposing ends of each control gate structures 109 a, 109 b over source/drain regions 138 a, 138 b, respectively. A dielectric 114 is seen between the gates of the flash memory cells. The source/drain regions 106, 138 a, 138 b have an electrical conductivity that is different from that of channel regions in Si substrate 102.

A HKMG transistor 136, which is illustrated at an intermediate stage in the manufacturing process, resides on top of the un-recessed substrate region 103. Touching the un-recessed substrate surface 102 a is a high-k dielectric layer 118 and above that, an etch-stopping layer 120 is disposed. A sacrificial gate 122 is formed above the metal etch-stopping layer 120. A dielectric hard mask 126 resides above the sacrificial gate 122 and sidewall spacers 124 a and 124 b are disposed on either side of the sacrificial gate 122. An inter-layer dielectric (ILD) 128 is deposited entirely over the devices, covering their top surfaces as well as surrounding them.

As will be appreciated in greater detail below, forming the memory cells 132, 134 on the recessed Si substrate region 101 (having the top surface 102 b) helps ensure the memory cells are disposed at a lower height level than the HKMG circuitry, such that the top surfaces of the pair of control gates 110 a and 110 b are co-planar with the top surface of the sacrificial gate 122 (as shown by horizontal line 130). Thus, when a CMP process is performed on an ILD layer 128 to stop at the level of horizontal line 130, the CMP process will not destroy or overly erode the CGs 110 a and 110 b. For comparison, in previous processes where the memory cells and HKMG transistor were formed on a planar substrate (i.e, when recessed Si substrate region 101 was not utilized), the CGs 110 a, 110 a would tend to “stick up” higher over the upper surface of sacrificial gate 122, and hence the CMP process would tend to “chop off” the top portion of the CGs 110 a, 110 b and thereby damage the memory cells 132, 134. The use of the recessed and un-recessed regions 101, 103 reduces this damage.

Isolation zone 104 c, which is formed in the transition region 105 and which has an asymmetric geometry about the y-axis, is a signature feature for ESF3 memories embedded on replacement gate HKMG technologies and it can be utilized as a distinguishable feature for various imaging processes. Two isolation zones that are symmetric about a y-axis are also illustrated as 104 a and 104 b. Symmetric isolation zone 104 a resides on the recessed Si region 101 while symmetric isolation zone 104 b resides on the un-recessed Si region 103.

FIG. 2 illustrates a flow diagram of some embodiments of a method 200 for forming an integrated circuit according to the present disclosure. While disclosed method 200 (and other methods described herein) is illustrated and described below as a series of acts or events, it will be appreciated that the illustrated ordering of such acts or events are not to be interpreted in a limiting sense. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. In addition, not all illustrated acts may be required to implement one or more aspects or embodiments of the description herein. Further, one or more of the acts depicted herein may be carried out in one or more separate acts and/or phases.

At 202, a substrate with a planar top surface is provided. In some embodiments, the substrate comprises Si.

At 204, a section of the substrate is recessed to a depth below the planar top surface, resulting in the formation of recessed and un-recessed surfaces that are horizontally disposed on the substrate.

At 206, a pair of memory cells is formed over the recessed surface of the substrate. In some embodiments, the memory cells comprise split gate flash memory cells.

At 208, a HKMG circuitry is formed over the un-recessed surface of the substrate. In some embodiments, the HKMG circuitry comprises a static random access memory (SRAM) cell, an input/output cell or a core cell.

FIG. 3 illustrates a flow diagram of some embodiments of a detailed method for forming an integrated circuit until a certain stage of its formation, according to the present disclosure. While disclosed method 300 is illustrated and described below as a series of acts or events, it will be appreciated that the illustrated ordering of such acts or events are not to be interpreted in a limiting sense. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. In addition, not all illustrated acts may be required to implement one or more aspects or embodiments of the description herein. Further, one or more of the acts depicted herein may be carried out in one or more separate acts and/or phases.

At 302, a Si substrate with a planar top surface is provided.

At 304, a section of the Si substrate is recessed to a depth below the planar top surface, to form recessed and un-recessed surfaces on the Si substrate.

At 306, active regions separated by isolation zones are formed within the entire Si substrate. In some embodiments, the isolation zones comprise shallow trench isolation (STI) regions filled with a dielectric material. The isolation zone within the transition region between the recessed and the un-recessed surface comprises an asymmetrical geometry.

At 308, a memory control gate is formed over the recessed surface on the Si substrate. The memory control gate is formed over a floating gate.

At 310, a sacrificial gate is formed over the un-recessed surface on the Si substrate. The sacrificial gate is formed over a high-k dielectric layer. In some embodiments, the top surface of the sacrificial gate and the memory control gate are co-planar.

At 312, an ILD layer if formed over the entire Si substrate. In some embodiments, the ILD layer is an oxide.

At 314, a CMP operation is performed on the ILD layer which stops at the top surface of the sacrificial gate and hence at the top surface of the memory control gate.

FIGS. 4A-4K illustrate embodiments of cross sectional images of the step by step method 300 for the formation of a recessed substrate surface with isolation zones, according to the present disclosure.

FIG. 4A illustrates a cross sectional image of a semiconductor body 400 a having an oxide layer 404 over a Si substrate 402. A silicon nitride (SiN) layer 404 is deposited over the entire oxide layer 404. In some embodiments, the thickness of the oxide layer 404 is 30 Angstroms and the thickness of the SiN layer 404 is 200 Angstroms.

FIG. 4B illustrates a cross sectional image of a semiconductor body 400 b, where the oxide layer 404 and the SiN layer 406 is removed from a defined region 408 from the Si substrate 402. The defined region 408 is the memory cell region and in some embodiments, the oxide layer 404 and the SiN layer 406 is removed using photo lithography.

FIG. 4C illustrates a cross sectional image of a semiconductor body 400 c, where an oxide 410 is grown over the Si substrate 402 in the defined region 408. In some embodiments, the oxide 410 is grown by wet oxidation.

FIG. 4D illustrates a cross sectional image of a semiconductor body 400 d, after removing the oxide 410 from the Si substrate 402, thereby creating a recessed region over the substrate 402. In some embodiments the oxide 410 is removed by wet etching or by a combination of wet etching and dry etching. In some embodiments an oxide thickness ranging between 400-1000 Angstroms is removed to create a recess depth ranging between 150-800 Angstroms.

FIG. 4E illustrates a cross sectional image of a semiconductor body 400 e, where the remaining oxide layer 404 and the SiN layer 406 are removed from the Si substrate. 412 a represents a recessed Si substrate surface or a memory cell region and 412 b represents an un-recessed Si substrate surface or a HKMG circuitry region. In some embodiments, the recessed Si substrate surface 412 a can be formed by dry etching a defined region that needs to be recessed. In some embodiments, the dry etching is followed by wet oxidation of the recessed Si substrate to prevent damage to the recessed Si substrate 412 a.

FIG. 4F illustrates a cross sectional image of a semiconductor body 400 f, where an oxide layer 414 is formed over the entire recessed and un-recessed Si substrate 402, followed by the deposition of another SiN layer 416. In some embodiments, the oxide layer 414 comprises a thickness of 90 Angstroms and the SiN comprises a thickness of 800 Angstroms.

FIG. 4G illustrates a cross sectional image of a semiconductor body 400 g, after forming active regions and shallow trenches for isolations zones. In some embodiments, the shallow trenches are formed by forming a mask over the SiN layer, the mask covering some portions of the SiN layer while leaving other regions of the SiN layer exposed and performing a dry etch to remove the exposed portions of the SiN layer and the corresponding Si layer underneath. The shallow trench 418 a is formed in the recessed region of the substrate 402 and the shallow trench 418 b is formed in the un-recessed region of the substrate 402. Trench 418 c is formed in the transition region between the recessed and the un-recessed surfaces and hence it has a different geometry from the other trenches. Trenches 418 a and 418 b are symmetrical along a vertical axis or a y-axis whereas trench 418 c is asymmetrical along y-axis.

FIG. 4H illustrates a cross sectional image of a semiconductor body 400 h, after depositing a shallow trench isolation (STI) oxide 420 over the entire Si substrate.

FIG. 4I illustrates a cross sectional image of a semiconductor body 400 i, after performing a CMP process on the STI oxide 420 layer. The CMP stops at the top surface of SiN layer 416 that is over the un-recessed surface. This leaves a layer of the STI oxide 420 above the SiN layer 416 in the recessed surface.

FIG. 4J illustrates a cross sectional image of a semiconductor body 400 j, after performing a wet etching to control the step height of the STI oxide 420. In some embodiments, wet etching comprises a dip in dilute hydrogen fluoride (HF).

FIG. 4K illustrates a cross sectional image of a semiconductor body 400 k, after removing the SiN layer 416 from the entire substrate. This will leave the Si substrate 402 with STI zones 418 a, 418 b and 418 c and active regions with a thin layer of oxide 414 on their top surfaces.

FIGS. 5A-5D illustrate cross sectional images of an integrated circuit during different stages of a replacement gate process, according to the present disclosure.

FIG. 5A illustrates a cross sectional image of a semiconductor body 500 a, at a certain stage of processing. At this stage, ion well implants or source/drain implants 516, 518 and 520 are formed in the substrate 402. A pair of adjacent split gate flash memory cells 502 and 504 has been formed on the recessed substrate surface, where the memory cell 502 comprises a thicker CG 506 a over a wider yet thinner FG 508 a, and memory cell 504 comprises a thicker CG 506 b over a thinner FG 508 b. A shared EG 510 is seen between each of the control gate floating stack, and SGs 512 a and 512 b on opposing side of each of the control gate, floating gate stack. A dielectric 514 is also formed between the gates.

At this stage, a HKMG transistor 522 is also under formation over the un-recessed surface of the Si substrate 402. A sacrificial gate stack comprising a high-k dielectric layer 526, an etch-stopping layer 528 and a sacrificial gate 530, is formed over the un-recessed surface of the substrate 402. Side wall spacer 524 a and 524 b are seen on either side of the sacrificial gate stack. A dielectric hard mask 519 is formed above the sacrificial gate 530. An ILD layer 532 is deposited over the entire substrate 402 which encompasses the entire integrated circuit. Due to the recession of the substrate in the memory cell region, the height of the CGs 506 a and 506 b and the sacrificial gate stack 530 will be controlled in such a way that their top surfaces are co-planar.

FIG. 5B illustrates a cross sectional image of a semiconductor body 500 b, after performing a CMP over the semiconductor body 500 a. The CMP process is designed to stop at the top surface of the sacrificial gate 530 and since the top surfaces of the CGs 506 a and 506 b are co-planar with the sacrificial gate 530, the CMP will not damage the CGs 506 a and 506 b of the memory cells 502 and 504.

FIG. 5C illustrates a cross sectional image of a semiconductor body 500 c, after removal of the sacrificial gate 530 from the HKMG transistor 522. An etch protection layer 534 is deposited on the top surfaces of the integrated circuit leaving only a region above the sacrificial gate 530 open. This etch protection layer 534 makes sure the rest of the device is not damaged during this replacement gate process. The etch-stop layer 528 comprises a metal and it protects the inside of the HKMG transistor 522 such that the poly silicon etch stops at the metal surface of the etch-stop layer 528.

FIG. 5D illustrates a cross sectional image of a semiconductor body 500 d, after formation of the metal gate 536. At this stage the replacement gate process is completed where a metal gate is deposited in the opening of the sacrificial gate 530. In some embodiments, the metal gate deposition comprises, depositing the metal gate layer entirely over the integrated circuit (with the etch-protection layer still covering the rest of the device), performing a CMP process to level the top the surface of the metal gate 536 and removing the protection layer 534.

It will be appreciated that while reference is made throughout this document to exemplary structures in discussing aspects of methodologies described herein that those methodologies are not to be limited by the corresponding structures presented. Rather, the methodologies (and structures) are to be considered independent of one another and able to stand alone and be practiced without regard to any of the particular aspects depicted in the Figs. Additionally, layers described herein, can be formed in any suitable manner, such as with spin on, sputtering, growth and/or deposition techniques, etc.

Also, equivalent alterations and/or modifications may occur to those skilled in the art based upon a reading and/or understanding of the specification and annexed drawings. The disclosure herein includes all such modifications and alterations and is generally not intended to be limited thereby. For example, although the figures provided herein, are illustrated and described to have a particular doping type, it will be appreciated that alternative doping types may be utilized as will be appreciated by one of ordinary skill in the art.

In addition, while a particular feature or aspect may have been disclosed with respect to only one of several implementations, such feature or aspect may be combined with one or more other features and/or aspects of other implementations as may be desired. Furthermore, to the extent that the terms “includes”, “having”, “has”, “with”, and/or variants thereof are used herein, such terms are intended to be inclusive in meaning—like “comprising.” Also, “exemplary” is merely meant to mean an example, rather than the best. It is also to be appreciated that features, layers and/or elements depicted herein are illustrated with particular dimensions and/or orientations relative to one another for purposes of simplicity and ease of understanding, and that the actual dimensions and/or orientations may differ substantially from that illustrated herein.

The present disclosure relates to a method of embedding a 3 ^(rd) generation super flash memory cell in a HKMG circuit that integrates a replacement gate technology of formation. The recessed substrate surface prepared for the memory cells will make sure the CGs of the memory cells are not damaged during the ILD CMP process, without sacrificing the features and dimensions of an ESF3 memory device. This method provides an asymmetrical STI zone in the transition region between the recessed and the un-recessed substrate surfaces which is a signature for this new methodology and this distinguished feature can be positively utilized during imaging techniques.

In some embodiments, the present disclosure relates to a method of forming an integrated circuit comprising, providing a substrate with a planar top surface, recessing a section of the substrate to a depth below the planar top surface to form recessed and un-recessed surfaces that are disposed horizontally on the substrate, forming a pair of memory cells over the recessed surface, and forming a high-k metal gate (HKMG) circuitry over the un-recessed surface.

In another embodiment, the present disclosure relates to a method of forming an integrated circuit comprising, providing a silicon (Si) substrate with a planar top surface, recessing a section of the Si substrate to a depth below the planar top surface to form recessed and un-recessed surfaces on the Si substrate, forming active regions separated by isolation zones within the entire Si substrate, forming a memory control gate over the recessed surface, forming a sacrificial high-k metal gate (HKMG) layer over the un-recessed surface, forming an inter-layer dielectric (ILD) layer over the entire Si substrate, and performing a chemical mechanical polishing (CMP) operation on the ILD layer, wherein the CMP operation stops to define top surfaces of the memory control gate for a memory cell and a sacrificial gate for a HKMG transistor.

In yet another embodiment, the present disclosure relates to an integrated circuit comprising, a substrate including a periphery region having a first substrate surface and a memory cell region having a second substrate surface, wherein the second substrate surface is recessed within the substrate relative to the first substrate surface, a high k metal gate (HKMG) transistor disposed on the first substrate surface and including a HKMG sacrificial gate and, two neighboring flash memory cells formed on the second substrate surface and including a pair of flash memory cell control gates, wherein top surfaces of the HKMG sacrificial gate and flash memory cell control gates are co-planar.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A method of forming an integrated circuit comprising: providing a substrate with a planar top surface; recessing a section of the substrate to a depth below the planar top surface to form recessed and un-recessed surfaces that are disposed horizontally on the substrate; forming a pair of memory cells over the recessed surface; and forming a high-k metal gate (HKMG) circuitry over the un-recessed surface.
 2. The method of claim 1, further comprising forming a transition region between the recessed and the un-recessed surfaces that extends into the substrate, having an asymmetrical geometry along an axis that is perpendicular to the planar top surface of the substrate.
 3. The method of claim 1, wherein: the memory cells comprise a pair of control gates; the HKMG circuitry comprises a sacrificial gate; and top surfaces of the pair of control gates and the sacrificial gate are co-planar.
 4. The method of claim 3, wherein forming the HKMG circuitry comprises a chemical mechanical polishing (CMP) process that stops at the top surfaces of the sacrificial gate as well as the pair of control gates.
 5. The method of claim 1, wherein the depth of the recessed surface, ranges from approximately 150 Angstroms to 800 Angstroms.
 6. A method of forming an integrated circuit comprising: providing a silicon (Si) substrate with a planar top surface; recessing a section of the Si substrate to a depth below the planar top surface to form recessed and un-recessed surfaces on the Si substrate; forming active regions separated by isolation zones within the Si substrate; forming a memory control gate over the recessed surface; forming a sacrificial gate (HKMG) over the un-recessed surface; forming an inter-layer dielectric (ILD) layer over the Si substrate; and performing a chemical mechanical polishing (CMP) operation on the ILD layer, wherein the CMP operation stops to define top surfaces of the memory control gate for a memory cell and a sacrificial gate for a HKMG transistor.
 7. The method of claim 6, wherein a first isolation zone formed in the recessed surface and a second isolation zone formed in the un-recessed surface have symmetric geometries along an axis that is perpendicular to the planar top surface of the Si substrate and a third isolation zone formed in a transition region between the recessed and the un-recessed surfaces, has an asymmetric geometry along the axis.
 8. The method of claim 6, wherein recessing a section of the Si substrate comprises: photo defining the section of the Si substrate that needs to recessed, to form a defined Si substrate surface; growing an oxide on the defined Si substrate surface through wet oxidation; and performing a first etch to remove the oxide grown on the defined Si substrate surface.
 9. The method of claim 8, wherein the first etch comprises a combination of dry etch and wet etch or a wet etch only.
 10. The method of claim 6, wherein recessing a section of the Si substrate comprises: photo defining the section of the Si substrate that needs to be recessed, to form a defined Si substrate surface; and dry etching the defined Si substrate surface to form the recessed surface.
 11. The method of claim 6, wherein forming active regions separated by isolation zones comprises: depositing a silicon nitride (SiN) layer over the Si substrate surface; forming a mask over the SiN layer, the mask covering some portions of the SiN layer while leaving other regions of the SiN layer exposed; performing a dry etch to remove exposed portions of the SiN layer and corresponding Si layer underneath; depositing an STI (shallow trench isolation) oxide over the Si substrate; polishing and leveling a step height of the STI oxide; and removing the SiN layer from the Si substrate.
 12. The method of claim 6, wherein forming a memory cell comprises: forming source and drain regions in the active regions; forming a pair of control gate structures comprising a thicker control gate over a wider yet thinner floating gate; forming an erase gate between two adjacent control gate structures; forming select gates on opposing sides of two adjacent control gate structures; and forming a dielectric layer between all the gates of the memory cells.
 13. The method of claim 6, wherein forming a HKMG transistor comprises: forming a high-k dielectric layer over the Si substrate; forming a metal etch-stopping layer over the high-k dielectric layer; forming a sacrificial polysilicon layer over the metal etch-stopping layer; patterning the sacrificial polysilicon layer and the layers underneath to form a sacrificial gate structure; forming an inter-layer dielectric (ILD) layer over and surrounding the sacrificial gate structure; performing the chemical mechanical polishing (CMP) on the ILD layer that stops at the top surface of the sacrificial polysilicon layer; removing the sacrificial polysilicon layer, thereby forming an opening in the gate structure; and forming a metal gate electrode in the opening, thereby forming a high-k metal gate.
 14. The method of claim 6, wherein the CMP operation does not damage top surfaces of the control gates of the memory cell.
 15. An integrated circuit comprising: a substrate including a periphery region having a first substrate surface and a memory cell region having a second substrate surface, wherein the second substrate surface is recessed within the substrate relative to the first substrate surface; a high k metal gate (HKMG) transistor disposed on the first substrate surface and including a HKMG sacrificial gate; and two neighboring flash memory cells formed on the second substrate surface and including a pair of flash memory cell control gates, wherein top surfaces of the HKMG sacrificial gate and flash memory cell control gates are co-planar.
 16. The integrated circuit of claim 15, further comprising a transition region arranged between the first substrate surface and the second substrate surface and extending into the substrate along an axis that is perpendicular to the first and second substrate surfaces, wherein the transition region is asymmetric about the axis.
 17. The integrated circuit of claim 15, wherein the two neighboring flash memory cells further comprises: a pair of control gate structures comprising a flash memory control gate disposed above a floating gate; a select gate disposed on opposing sides of each of the control gate structures; and an erase gate disposed between the control gate structures.
 18. The integrated circuit of claim 17, wherein the floating gates, control gates, erase gate and the sacrificial gate of the HKMG transistor comprise poly silicon.
 19. The integrated circuit of claim 15, wherein the first and second substrate surfaces comprise source/drain regions disposed within.
 20. The integrated circuit of claim 15, wherein the HKMG transistor comprises a static random access memory (SRAM) cell, an input/output cell or a core cell. 